1. Field of the Invention
The present invention relates to a method for satisfactorily monitoring the depth of an impurity implanted in a wafer upon manufacture of a semiconductor device.
2. Description of the Related Art
Upon manufacture of a semiconductor device, an ion implantation device (not shown), which implants ion species (hereinafter called simply “impurity”) of impurities such as phosphorous (P), Arsenic (As), Boron, etc., needs to implant an impurity to a depth, which is given at a distance measured from a main surface of the wafer on the execution side of ion implantation and which is as expressed in a predetermined set value.
However, when the ion implantation device continues to operate for a long period, it might implant an impurity to a depth different from the set value. Such a phenomenon principally takes place due to the fact that malfunctions occur in each part of the ion implantation device. Particularly when the impurity is accelerated so that needless discharge occurs in a part (hereinafter called acceleration part) injected into a wafer, injection energy E varies and hence such a phenomenon is apt to take place. There is a need to detect in early stages such a phenomenon because the characteristic of the wafer greatly varies when the thickness of a conductive layer of the wafer is thin in particular.
Thus, each semiconductor device maker has monitored the depth (hereinafter called “implantation depth”) of an impurity implanted in a wafer in the following manner.
That is, upon fabrication of a semiconductor device, the ion implantation device being in operation is stopped on a regular basis to thereby introduce a measuring wafer for measuring the implantation depth of the impurity onto a production line in place of a manufacturing wafer provided for the manufacture of the semiconductor device, which has been introduced onto the production line till then. And each semiconductor device maker operates the ion implantation device again to effect processing to be described below with reference to FIG. 10 on the measuring wafer. Incidentally, the measuring wafer is a wafer in which in order to accurately measure the impurity implantation depth, the type of material for each layer constituting the wafer, and the accurate thickness of each layer have been determined in advance. The measuring wafer might be identical to or different from the manufacturing wafer in constitution.
FIG. 10 is a view for describing a processing process effected on a conventional measuring wafer. Respective process steps are shown in FIGS. 10A through 10C.
In the example shown in FIG. 10, a measuring wafer 10 has a configuration in which an oxide film (e.g., SiO2) 14 is formed on a silicon wafer (hereinafter called “Si wafer”) 12. Incidentally, a main surface of the measuring wafer 10 functions as an exposed upper surface 14a of the oxide film 14. An impurity 70 is implanted from above the upper surface 14a of the oxide film 14.
In the measuring wafer 10, the thickness of each layer has been determined in advance as mentioned above. In the present example, the thickness H of the Si wafer 12 is assumed to range from approximately 100 Å to 1000 Å units (l×103 to m×104 Å) (where “l” and “m” are arbitrary integers), and the thickness h of the oxide film 14 is assumed to be about 100 Å. Incidentally, the Si wafer 12 is heat-treated after the impurity 70 has been implanted in the neighborhood of its surface. Thus, the neighborhood of the surface of the Si wafer 12 takes a diffused area of the impurity 70, thus resulting in a conductive layer. The oxide film 14 functions as a film for prevention of outward diffusion of the impurity 70.
The ion implantation device accelerates the impurity (P, for example) 70 at injection energy E and implants the accelerated impurity 70 in the measuring wafer 10 from above the upper surface 14a of the oxide film 14 (see FIG. 10A). Incidentally, the injection energy E is set in consideration of the thickness h of the oxide film 14 formed on the Si wafer 12 in such a manner that the impurity 70 is sufficiently implanted in the neighborhood of the surface of the Si wafer 12.
Each individual impurity 70 implanted in the measuring wafer 10 penetrates the oxide film 14 while its outward diffusion is being suppressed by the oxide film 14, and reaches the Si wafer 12. Each impurity 70 injected into the measuring wafer 10 collides with an internal crystal of the Si wafer 12, proceeds while being scattered, and stops at a point where the injection energy is used up (see FIG. 11). Incidentally, FIG. 11 is a view showing the process of intrusion of the impurity. At this time, if no malfunctions occur in respective parts (e.g., an acceleration part, etc.) of the ion implantation device, then most of the impurities 70 implanted in the measuring wafer 10 stop at a depth near a set value. If the malfunctions occur in the respective parts of the ion implantation device, then they stop at a depth different from the set value.
Thus, the ion implantation device forms an impurity-implanted layer 72 with the impurity 70 injected therein, in the vicinity of the surface of the Si wafer 12 (see FIG. 10B).
Incidentally, since the individual impurities 70 implanted in the measuring wafer 10 are affected by crystals in the measuring wafer 10, they are placed in a random order in the way of being diffused. Therefore, even though each individual impurity 70 is implanted at the same injection energy E, a variation width occurs in each stop position.
FIG. 12 is a view showing an impurity concentration profile. The horizontal axis of FIG. 12 indicates the concentration (cm−3) of the impurity 70, and the vertical axis thereof indicates an implantation depth of the impurity 70 as viewed from the main surface of the wafer.
If no malfunctions occur in the respective parts of the ion implantation device as well known, then the impurity 70 is distributed in Gaussian distribution and protruded forms with a set value RP of an implantation depth as the center as shown in FIG. 12. Thus, if no malfunctions occur in the respective parts (e.g., an acceleration part, etc.) of the ion implantation device, then the concentration of the impurity 70 becomes the highest at the set value RP of the implantation depth and becomes low as it falls outside the set value RP of the implantation depth.
Incidentally, the depth Rp in FIG. 12 shows a depth (hereinafter called “concentration peak depth”) at which the concentration of the impurity 70 at its actual measurement becomes the peak. Since no malfunctions occur in the respective parts of the ion implantation device in the example shown in FIG. 12, the concentration peak depth Rp coincides with the set value RP of the implantation depth. However, if the malfunctions occur in the respective parts of the ion implantation device, then the concentration peak depth Rp does not coincide with the set value RP of the implantation depth.
In FIG. 12, areas D with dots and a diagonally-shaped area S indicate areas set so as to become the impurity-implanted layer 72 shown in FIG. 10B in the Si wafer 12. The areas in the Si wafer 12, which are set so as to become the impurity-implanted layer 72, will hereinafter be called “target area” for implanting the impurity 70. The target area serves as the impurity-implanted layer 72 by the implantation of the impurity 70 therein. In the present example, upper and lower ends of the target area as taken in its depth direction are respectively set to a position where a concentration C1 of the impurity 70 reaches about 10% of a concentration Cp of the impurity 70 at the set value RP of the implantation depth. In FIG. 12, a depth Re1 shows the depth of the upper end of the target area, whereas a depth Re2 indicates the depth of the lower end of the target area. According to the conventional experiences, electric resistivity of each of external areas (i.e., an upper area shallower than the depth Re1 and a lower area deeper than the depth Re2) of the target area reaches ten or more times that of an internal area (i.e., an area below the depth Re1 and above the depth Re2) of the target area.
In FIG. 12, the diagonally-shaded area S indicates an area in which the impurity 70 is contained in a high concentration. This area S results in a conductive layer significant from a practical standpoint. In the present example, upper and lower ends of the area S as taken in its depth direction are respectively set to a position where a concentration C2 of the impurity 70 reaches about 50% of the concentration Cp of the impurity 70 at the set value RP of the implantation depth. According to the result of measurement, the area S normally results in an area which contains about ⅔ (about 68%) of the amount of the impurity 70 injected into the measuring wafer 10. As already mentioned above, variations occur in the implantation depth of the impurity 70. Therefore, the distance (i.e., the distance from the set value RP of the implantation depth to the upper end of the area S or the distance from the set value RP of the implantation depth to the lower end of the area S) from the set value RP of the implantation depth to the upper or lower end of the area S is taken as a width at which the impurity 70 varies in upper and lower depth directions with the set value RP of the implantation depth as the center. This is particularly referred to as a diffusion variation width ΔRp as viewed in the depth direction, of the impurity 70. Thus, the area S is brought to an area of (RP±ΔRp) with the set value RP of the implantation depth as the center.
If no malfunctions occur in the respective parts (especially, the acceleration part), then the concentration peak depth Rp and the diffusion variation width ΔRp are determined by the type of impurity 70, the dose of the impurity 70, the type of target intended for implantation of the impurity 70, and injection energy E. For example, the type of impurity 70 is assumed to be phosphorous (P), the dose of the impurity 70 is assumed to be 5×1015cm−2, the type of target intended for implantation of the impurity 70 is assumed to be of the Si wafer 12 (however, on condition that the number of defects on its surface is less than or equal to a predetermined value defined in advance), and the injection energy E is assumed to be 50 keV. At this time, according to the pre-executed result of measurement, the concentration peak depth Rp results in about 600 Å and the diffusion variation width ΔRp results in about 280 Å. However, if the malfunctions occur in the respective parts of the ion implantation device, then the concentration peak depth Rp and the diffusion variation width ΔRp result in values different from them.
Incidentally, if the respective parts of the ion implantation device malfunction, then the substantial dose (i.e., dose in the target area set so as to be the impurity-implanted layer 72, which is placed between the depths Re1 and Re2) of the impurity 70 is reduced as compared with the case in which no malfunctions occur in the respective parts of the ion implantation device. Such a phenomenon occurs due to the fact that the concentration peak depth Rp is displaced from the set value RP of the implantation depth. Assuming that the dose of the impurity 70 in the target area where no malfunctions occur in the respective parts of the ion implantation device, is 100%, for example, the substantial dose of the impurity 70 is reduced to about 16% with only a displacement of the concentration peak depth Rp from the set value RP of the implantation depth, which displacement is equivalent to of about twice (i.e., about 560 Å) the diffusion variation width ΔRp, where the respective parts of the ion implantation device malfunction.
Next, the ion implantation device heat-treats the whole measuring wafer 10 at a predetermined temperature (e.g., about 950° C.) to activate the impurity 70 of the impurity-implanted layer 72. With the activation of the impurity 70, an impurity active layer 72 is formed from the impurity-implanted layer 72 (see FIG. 10C). This impurity active layer 74 serves as a conductive layer. And the ion implantation device removes the oxide film 14 from the Si wafer 12 to expose the impurity active layer 74. Incidentally, an unillustrated device other than the ion implantation device may be caused to perform the heat treatment and the removal of the oxide film 14.
Thereafter, an unillustrated measurement device is operated to measure surface resistivity of the measuring wafer 10 by a four probe method or the like.
The surface resistivity Rs of the measuring wafer 10 is equivalent to electric resistivity with Ω/sq as the unit and may be referred to as “sheet resistance”. Incidentally, the surface resistivity Rs of the measuring wafer 10 can be provided by a pre-executed measurement with the relationship with the concentration peak depth Rp of the impurity 70 as Table. That is, the surface resistivity Rs of the measuring wafer 10 can be provided by taking, as Table, a relationship as to what value reaches the concentration peak depth Rp of the impurity 70 according to to what extent the value of the surface resistivity Rs corresponds.
The surface resistivity Rs of the measuring wafer 10 decreases as the substantial dose of the impurity 70 increases, and increases as the substantial dose thereof decreases. There may be cases in which the substantial dose of the impurity 70 depends on the concentration peak depth Rp of the impurity 70 and diffusion of the impurity 70 by heat treatment. The substantial dose of the impurity 70 dependent on the former is referred to as “substantial dose (or first substantial dose) dependent on the concentration peak depth”. Further, the substantial dose of the impurity 70 dependent on the latter is called “substantial dose (or second substantial dose) dependent on the heat treatment”.
When the concentration peak depth Rp of the impurity 70 reaches the set value RP of the implantation depth, the surface resistivity Rs of the measuring wafer 10 becomes the minimum. This is because the first substantial dose of the impurity 70 reaches the maximum.
On the other hand, as the concentration peak depth Rp of the impurity 70 falls outside the set value RP of the implantation depth, the surface resistivity Rs of the measuring wafer 10 increases. This is because the impurity 70 is partly implanted in an area (i.e., area deeper than the neighborhood of the surface of the Si wafer 12, or the oxide film 14) out of the neighborhood of the surface of the Si wafer 12, so that the first substantial dose of the impurity 70 decreases. Since the thickness of the Si wafer 12 is thick even though the concentration peak depth Rp of the impurity 70 falls outside the set value RP of the implantation depth, the implanted impurity 70 is injected into an internal area (i.e., area deeper than the neighborhood of the surface of the Si wafer 12) of the Si wafer 12 for the most part. Further, the impurity 70 implanted in the area is partly diffused due to the influence of heat treatment and forms a conductive layer. Therefore, the second substantial dose of the impurity 70 increases. Thus, the substantial dose of the impurity 70 is not so reduced because the amount of an increase in the second substantial dose is added to the amount of a decrease in the first substantial dose. As a result, the surface resistivity Rs of the measuring wafer 10 increases since the concentration peak depth Rp of the impurity 70 falls outside the set value RP of the implantation depth but is relatively small in the amount of its increase.
When the measurement device measures the surface resistivity Rs of the measuring wafer 10, it compares the surface resistivity Rs and an allowable value set in advance and determines whether the concentration peak depth Rp of the impurity 70 falls within an allowable range. Incidentally, the allowable value is equivalent to the value of surface resistivity Rs corresponding to the concentration peak depth Rp where the distance of the concentration peak depth Rp from the set value RP of the implantation depth reaches a limit allowable as a displacement width. This allowable value is suitably set according to operations. When the surface resistivity Rs is of the allowable value or less, the measurement device determines that the concentration peak depth Rp falls within the allowable range. In this case, each semiconductor device maker determines that the concentration peak depth Rp remains unchanged. On the other hand, when the surface resistivity Rs is larger than the allowable value, the measurement device determines that the concentration peak depth Rp falls outside the allowable range. In this case, each semiconductor device maker determines that the concentration peak depth Rp falls outside the set value RP of the implantation depth, and resets the state of the ion implantation device.
Each semiconductor device maker has monitored the implantation depth of the impurity 70 by the above-mentioned method (refer to, for example, a patent document, i.e., Japanese Patent Application Laid-Open No. 2003-151913 (third to fifth paragraphs, eighteenth to fiftieth paragraphs, and FIGS. 1 through 10)).
Meanwhile, the implantation depth of the impurity 70 couldn't be monitored so far satisfactorily due to the following reasons.
(1) The surface resistivity Rs of the measuring wafer 10 varies due to the fact that the implantation depth (concentration peak depth Rp) of the impurity from the main surface of the wafer falls outside the set value RP of the implantation depth. However, a variation in the implantation depth of the impurity 70 couldn't be detected accurately up to now since the amount of an increase in the surface resistivity Rs was relatively small.
(2) The surface resistivity Rs of the measuring wafer 10 varies even due to the fact that the concentration profile of the impurity 70 changes due to the influence of heat treatment. Thus, the surface resistivity Rs of the measuring wafer 10 changes even by being affected by factors irrelevant to the injection energy E quantitatively given to the impurity 70 by the ion implantation device. Therefore, the variation in the implantation depth of the impurity 70 could not be detected accurately up to now.
Since the first substantial dose of the impurity 70 becomes the maximum when the concentration peak depth Rp of the impurity 70 is in the vicinity of the set value RP of the implantation depth as mentioned above, for example, the surface resistivity Rs of the measuring wafer 10 becomes the minimum. On the other hand, as the concentration peak depth Rp of the impurity 70 falls outside the set value RP of the implantation depth, the first substantial dose of the impurity 70 decreases and hence the surface resistivity Rs of the measuring wafer 10 increases. Due to the fact that the concentration peak depth Rp of the impurity 70 falls outside the set value RP of the implantation depth in this way, the first substantial dose of the impurity 70 changes and correspondingly the surface resistivity Rs of the measuring wafer 10 changes.
When the concentration profile of the impurity 70 changes due to the influence of heat treatment as described above, the second substantial dose of the impurity 70 increases and hence the surface resistivity Rs of the measuring wafer 10 decreases. Due to the fact that the concentration profile of the impurity 70 changes due to the influence of heat treatment in this way, the second substantial dose of the impurity 70 changes and correspondingly the surface resistivity Rs of the measuring wafer 10 changes.
If the surface resistivity Rs of the measuring wafer 10 changes, it is then unclear to what extent which of the change in the first substantial dose of the impurity 70 and the change in the second substantial dose of the impurity 70 influences the amount of its change. That is, it is not clear to what extent the amount of change in the surface resistivity Rs increases due to the fact that the concentration peak depth Rp of the impurity 70 falls outside the set value RP of the implantation depth or to what extent the amount thereof decreases due to the fact that the concentration profile of the impurity 70 changes. Therefore, the variation in the implantation depth of the impurity 70 could not be detected so far.
(3) Particularly when the injection energy E is relatively small, the surface resistivity Rs of the measuring wafer 10 is affected by factors irrelevant to the injection energy E quantitatively given to the impurity 70 by the ion implantation device and is liable to variation. Therefore, when the injection energy E is relatively small in particular, the variation in the implantation depth of the impurity 70 could not be detected accurately by only the quantitative factors such as the injection energy E.
When, for example, many defects occur in the surface of a target to be implanted, the concentration peak depth Rp of the impurity 70 is apt to fall outside the set value RP of the implantation depth according to the number of the defects. Therefore, the first substantial dose of the impurity 70 is also liable to change according to the number of the defects in the surface of the target to be implanted. Further, the surface resistivity Rs of the measuring wafer 10 is also apt to change according to the change in the first substantial dose of the impurity 70. Thus, the surface resistivity Rs of the measuring wafer 10 is liable to change according to the number of defects in the surface of the target to be implanted, that is, with being affected by the factors irrelevant to the injection energy E quantitatively given to the impurity 70 by the ion implantation device. Since the concentration peak depth Rp of the impurity 70 is greatly affected by the number of the defects in the surface of the target to be implanted where the injection energy E is relatively small in particular, such a phenomenon noticeably appears. Therefore, a variation in the implantation depth of the impurity 70 could not be detected accurately so far with only the quantitative factors such as the injection energy E particularly when the injection energy E was relatively small.
Incidentally, the variation in the implantation depth of the impurity 70 can be monitored even by measuring the number of defects in the surface of the measuring wafer 10. However, the present method was not capable of accurately detecting the variation in the implantation depth of the impurity 70 either due to the above reasons (1) through (3) in a manner similar to the monitoring of the surface resistivity Rs of the measuring wafer 10 by measurement. Since the influence suffered from the number of the defects in the surface of the measuring wafer 10 differed according to the type of impurity 70 in particular, the present method could not detect the variation in the implantation depth of the impurity 70 accurately with only the quantitative factors such as the injection energy E.
As described above, the surface resistivity Rs of the measuring wafer 10 is relatively small in the amount of its increase even though the concentration peak depth Rp of the impurity 70 falls outside the set value RP of the implantation depth. Further, the surface resistivity Rs thereof changes even by being affected by the factors irrelevant to the injection energy R quantitatively given to the impurity 70 by the ion implantation device. Therefore, the implantation depth of the impurity 70 could not be monitored satisfactorily so far. Particularly when the injection energy E is relatively small, the variation in the implantation depth of the impurity 70 could not be detected accurately with only the quantitative factors such as the injection energy E.